The present invention relates generally to workpiece characterization systems and methods of use. More particularly, the present invention relates to a system, method and software program product for exciting production light emitting diodes with a wide spectrum excitation light source capable of exciting a light emitting diode without interfering with photoluminescence emission light emitted there from.
Workpiece characterization systems are employed in a variety of industries, such as the semiconductor processing industry, for real-time and/or near-real-time monitoring of workpiece properties, modification and process control. Workpiece characterization systems may be integrated with a semiconductor processing tool and utilized in-situ for real-time process control or may be used in-line for feedback/feedforward control.
Due to the rapid advancement of the use of light emitting diodes (“LEDs”) as energy efficient and “green” lighting technologies, characterization and yield control/analysis for LEDs has seen intense demand as market forces drive product reliability up and costs down. For LED product wafers, yields must increase from their current levels to achieve industry-targeted cost levels. Yield loss in LEDs may arise in the forms of low output, decreased lifetime, shifted wavelength output and other properties. Many of the properties are not evaluated until LED product wafers are diced and sorted. With a long delay between LED wafer fabrication and LED property evaluation, correction of process drifts, excursion and other drivers of yield loss may not be corrected quickly enough leading to inefficiencies in wafer processing. For lighting applications LED output and color are important factors since the human eye may detect wavelength shifts as small as 1 nm at blue-green wavelengths and LEDs require color sorting for applications such as backlighting and general illumination to provide uniformity.
A main historical method for optical characterization of LEDs has been the use of photoluminescence which is the absorption and re-emission of photons by a material. Photoluminescence provides a rapid and non-contact method for determination of many parameters that affect yield. For LEDs of current market interest for lighting applications, especially Gallium Nitride (“GaN”) and related alloys of Aluminum and Indium, UV light is used to excite photoluminescence of longer wavelengths of UV/Blue emission which is commonly phosphor converted to provide “white light.”
FIG. 1 shows a pictorial schematic of a prior art workpiece characterization system 100. Workpiece characterization system 100 includes excitation source 110 which emits light 115 directed through optics 120, to be incident at angle Θ1 on workpiece 130. Photoluminescence emission light 140 derived from excitation of workpiece 130 is guided through optics 150 to light analyzing device 160 oriented at measurement angle Θ2. Excitation source 110 is commonly a narrowband emission source such as a laser. Optics 120 and 150 may include any number of lenses, mirrors, filters or other optical elements necessary to transform light passing from excitation source 110 to workpiece 130 and/or from workpiece 130 to light analyzing device 160. Light analyzing device 160 is commonly a spectrograph, spectrometer, monochromator, photodiode, photomultiplier tube (“PMT”) or other light analyzing device providing wavelength discrimination. By configuring workpiece characterization system 100 such that the incident and reflected angles, Θ1 and Θ2 respectively, are non-equal; saturation and or contamination of the photoluminescence emission light 140 by specularly scattered excitation light 117 is avoided.
The aforedescribed workpiece characterization system 100 presents multiple limitations which are discussed herein below. The present invention seeks to mitigate the short-comings of the prior art and provide systems and methods for rapid analysis of LED product wafers inline or in-situ enabling improved yield.